U.S. Pat. No. 3,190,987 to A. H. Fister, and assigned to the assignee of the present invention, describes what is now commonly referred to as a GMT fuse or a "grasshopper fuse". Another such fuse is disclosed in U.S. Pat. No. 4,496,929, and is also assigned to the assignee of the present invention. These fuses are commonly used in telecommunications, computer or control circuits. One special feature of these fuses is that they provide visual spring indicating and alarm circuit indicating capabilities. Basically, such a fuse incorporates a resilient contact that is normally held out of engagement with an adjacent alarm contact by the fusible element of that fuse and that moves into engagement with the alarm contact when that fusible element "blows". These fuses are manufactured by the Bussmann Division of Cooper Industries in connection with HLT, HLS, HWT, and PCT fuse holders.
These fuses are used in combination with a fuse carrier which is made rugged and strong so that the fuse can be grasped to facilitate its insertion into and its removal from a recess within the fuse holder for that fuse. The fuse carrier has terminals secured thereto which are electrically connected only through the medium of the fuse element of that fuse and only in the event that the fusible element "blows".
The terminals of the fuse carrier are adapted to be received and held by terminals in a fuse holder designed for that fuse. The fuse holder typically has a recess in which part of the fuse carrier can extend with the walls of that recess serving to guide or position the terminals of the fuse holder.
The terminals in the fuse holder are generally U-shaped in elevation and have short confronting faces which are convex. The upper and lower ends of the terminals abut and are held against outward movement by portions of the inner faces of that fuse holder. The overall result is that the confronting faces of the arms of the terminals of the fuse holder are resistant to bending and will thus firmly grip and hold the terminals of the fuse carrier of the fuse. The terminals are held to the fuse holder by means of a shank which extends through an opening in the closed end of the aforesaid recess and are then twisted to prevent separation of those terminals from the fuse holder.
Operating experience has shown that there are some short comings with the design just described. The convex confronting faces of the terminal essentially results in one point contact per side of the fuse clip. In addition, the fuse clip, when assembled in a fuse holder with a fuse installed, was found to be susceptible to a widely changing electric resistance at the fuse clip-contact interface, when subjected to shock or vibration. Part of the reason may be that clip length is relatively short and fuse contact geometry was not always the same. U.S. Pat. No. 4,643,510 to Urani describes one attempt to improve upon the situation by providing a fuse clip with two points of contact on each side of the fuse; it too is a short clip.
There are other problems. High insertion forces are experienced when inserting the fuse into the fuse holder as a result of the fuse clip being relatively short in length and having very little "spring". The insertion force can be lowered by making the clips from a soft material, such as brass. If made from a beryllium-copper alloy, a post-forming, averaging, heat treatment is performed on the fuse clips (i.e., typically done at 800 degrees F. for two hours in the case of a beryllium-copper clip). This heat treatment increases electrical conductivity slightly, but lowers the tensile strength and reduces the mechanical resistance of the clip. In either case, clips were found to take on a "set" when fuses were inserted and extracted many times. Moreover, if the above-described heat treatment was used, it was found that this time and temperature combination was not easy to control, unless it was watched very closely. Thus, there is a need for an improved fuse clip which does not have these short comings.